In recent years, an organic electroluminescence (Organic Electro-Luminescence, it is called “organic EL”) device attracts attention as a light emitting device. The organic electroluminescence device (hereinafter, it is abbreviated as “organic EL device”) has a characteristic of superior response because a time from start up of power distribution to light emission is very short and brightness instantly varies by varying electric current. In addition, the organic EL device has characteristics that the response hardly varies with temperature, and an angle of field of view thereof is near to 180 degrees. Due to these characteristics, the organic EL device is suitable to surface emission, and thus, it is utilized to a lighting appliance such as a backlight of a liquid crystal display apparatus, recently.
As for a light control apparatus for the organic EL device, one having light control function to control the brightness (brilliance) of the organic EL device is proposed as described in JP2006-210848A or JP2004-245904A, for example. FIG. 11 shows a circuit diagram of a lighting appliance which comprises a light source 1 consisting of an organic EL device and a light control apparatus 2 for lighting the light source 1. The light control apparatus 2 supplies a constant electric current of a predetermined value to the light source 1 consisting of the organic EL device, and comprises a rectification circuit 3 consisting of a diode bridge to perform full wave rectification to an alternating electric current which is an output of an AC power supply such as a commercial power source, a power-factor correction circuit unit (hereinafter, it is abbreviated as “PFC circuit”) 4 which converts the alternating electric current output performed the full-wave rectification by the rectification circuit 3 to a direct electric current output, an electric current output circuit 5 which outputs a constant electric current for lighting the light source 1 on the basis of the direct electric current output from the PFC circuit 4, and a light control circuit 6 which performs light control of the light source 1 by intermittently supplying the constant electric current outputted from the electric current output circuit 5 to the light source 1.
The PFC circuit 4 has a coil (a choke coil) L1, an end of which is connected to an output terminal of a high electric potential side of the rectification circuit 3. A series circuit of a switching element Q1 such as a MOSFET of n-channel type and a resistor R1 is set between another end of the coil L1 and an output terminal of a low electric potential side of the rectification circuit 3. An anode of a diode D1 for preventing counter electric current is connected to a connection point of the coil L1 and the switching element Q1. A smoothing capacitor C1 such as an electrolytic capacitor is set between a cathode of the diode D1 and the output terminal of the low electric potential side of the rectification circuit 3.
In such PFC circuit 4, when switching on the switching element Q1, an electric current flows from the output terminal of the high electric potential side of the rectification circuit 3 through the coil L1 and the switching element Q1, so that an energy is accumulated in the coil L1. Subsequently, when switching off the witching element Q1, the energy accumulated in the coil L1 is supplied to the smoothing capacitor C1 through the diode D1. By repeating switching on and off of the switching element Q1, supplying and cutting off of the electric current to the capacitor C1 is repeated. By perform the on/off control of the switching element Q1 with a high frequency, a voltage between both terminals of the capacitor C1 can be set to a desired value.
The switching element Q1 is ON/OFF controlled by a PFC control unit 4a. The PFC control unit 4a detects presence or absence of an electric current flowing in the coil L1 by a coil L2 and detects a source voltage of the switching element Q1, and performs the ON/OFF control of the switching element Q1 based on the detected value of the electric current so as to maintain the voltage between the both terminals of the capacitor C1 constant. In addition, the PFC control unit 4a comprises a power-factor correction function to correct distortion of harmonic component of the electric current obtained from the AC power supply, and sets an ON term of the switching element Q1 to correct the power-factor of input of the AC power supply, for example.
The electric current output circuit 5 is a step-down chopper circuit which depresses a voltage of the direct electric current output of the PFC circuit 4, and serves as a constant electric current source which outputs a constant electric current to the light source 1. The electric current output circuit 5 comprises: a switching element Q2 such as a MOSFET of n-channel type, a drain of which is connected to the high electric potential side of the smoothing capacitor C1 of the PFC circuit 4; a coil L3, an end of which is connected to a source of the switching element Q2; a diode D2 for backflow, an anode of which is connected to the low electric potential side of the smoothing capacitor C1; a smoothing capacitor C2 such as an electrolytic capacitor connected to another end of the coil L3; a resistor R2 set between the low electric potential side of the capacitor C2 and an anode of the diode D2; and an electric current control unit 5a which performs ON/OFF control of the switching element Q2. By performing the ON/OFF control of the switching element Q2, a voltage between both ends of the capacitor C2, that is, the voltage applied to both ends of the light source 1 can be set to a desired value. The electric current control unit 5a performs the ON/FF control of the switching element Q2 based on a voltage between both ends of the resistor R2 so as to make the voltage between the both ends of the smoothing capacitor C2 constant, and thus, the electric current output circuit 5 outputs a constant electric current to the light source 1.
The light control circuit 6 is configured of a switching element Q3 such as a MOSFET of n-channel type, a drain of which is connected to a high electric potential side of the capacitor C2 of the electric current output circuit 5 and a light control unit 6a which performs the ON/OFF control of the switching element Q3. The light control unit 6a performs the ON/OFF control of the switching element Q3 in a high frequency by applying voltages to a gate of the switching element Q3 as shown in FIG. 12A based on a given duty ratio. Consequently, electric currents are intermittently outputted to the light source 1 as shown in FIG. 12B, so that light control of the light source 1 is performed.
By the way, an equivalent circuit of the organic EL device which is the light source 1 is represented by a parallel circuit of a diode and a capacitor. In other words, the organic EL device is a capacitive load having a capacitive component. Therefore, when supplying of electric power to the light source 1 is started by switching on the switching element Q3 in the light control apparatus 2 of the conventional case, a surge electric current (a charge electric current) flows in the organic EL device to charge the capacitive component of the organic EL device, so that stress is given to the organic EL device (hatched regions designated by symbols P1 in FIG. 12B). In addition, since the organic EL device does not emit light until the capacitive component is charged, rising-up of light flux outputted from the light source 1 is delayed. On the other hand, when the switching element Q3 is switched off and supplying of electric power to the light source 1 is stopped, electric charge accumulated in the capacitive component of the organic EL device is discharged (hatched regions designated by symbols P2 in FIG. 12B). Therefore, a surge electric current flows to charge the capacitive component of the organic EL device again, when the switching element Q3 is switched on subsequently.
In this way, since charge and discharge of the capacitive component of the organic EL device serving as the light source 1 are performed when the switching element Q3 is switched on and off, electric power supplied to the light source 1 is consumed wastefully, and luminous efficiency becomes worse. Such problems are significant when the switching element Q3 is switched on and off in a high frequency.